A very long time ago, I thought "one day at a time" was a bumper sticker phrase of encouragement. Years later I learned the actual source: the author is Christ.
“Look at the birds of the air, that they do not sow, nor reap...and yet your heavenly Father feeds them. Are you not worth much more than they?” (Matthew 6:25-34)
He begins by talking to his disciples, instructing them how to pray sincerely and not like the hypocrites. He taught them the model prayer (Matthew 6:7-15).
He explained forgiveness and righteousness.
He spoke about where they should store up their treasures, not on earth where moth and rust destroys them, but store them in heaven; for "where your treasure is, there your heart will be also." (Matthew 6:19-21)
Then he continued by reassuring them because they were anxious about what they would have to eat and drink, and what they would have to wear, and how their needs would be met.
"So do not worry about tomorrow; for tomorrow will care for itself. Each day has enough trouble of its own."
Live each day for itself, one day at a time.